Method and apparatus for gas flow control

ABSTRACT

A method and apparatus for self-calibrating control of gas flow. The gas flow rate is initially set by controlling, to a high degree of precision, the amount of opening of a flow restriction, where the design of the apparatus containing the flow restriction lends itself to achieving high precision. The gas flow rate is then measured by a pressure rate-of-drop upstream of the flow restriction, and the amount of flow restriction opening is adjusted, if need be, to obtain exactly the desired flow.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.14/475,494, filed Sep. 2, 2014, which is a divisional of U.S. patentapplication Ser. No. 12/906,058, filed Oct. 15, 2010, which claimspriority benefit from U.S. Provisional Patent Application No.61/252,143, filed Oct. 15, 2009, the disclosures of which areincorporated herein by reference in their entireties.

BACKGROUND

1. Field

This invention is in the field of fluid flow control and, morespecifically the field of high accuracy flow control such as requiredfor, e.g., semiconductor processing, plat panel display fabrication,solar cell fabrication, etc.

2. Related Art

Metering the mass-flow rate of a gas is important to many industrialprocesses. In the case of the semiconductor industry, metering must beespecially accurate, because deviations in the flow rate of only severalpercent can lead to process failures.

Mass flow is the result of a pressure gradient existing in a system. Aslong as no external work is done on the system, mass will flow fromareas of high pressure to low pressure. This is the working principle inall flow control devices. In order to control the rate of flow from thehigh pressure region to the low pressure region, a flow restriction isused. The flow restriction is positioned such that all flow in thesystem must pass through the restriction. Depending on thecharacteristics of the restriction, mass flow rate through the flowcontrol device is a function of some or all of the following: dimensionsof the flow restriction, the magnitude of the pressures both upstreamand downstream of the flow restriction, the temperature of the system,and the physical properties of the gas, such as density and dynamicviscosity. The flow rate can be controlled by varying one or more ofthese parameters. In general, the physical properties of the gas and thetemperature of the system are difficult to change or control, so flow iscontrolled by varying the pressures in the system, the dimensions of theflow restriction, or both.

The industry-standard flow control device is a mass flow controller(MFC) containing a flow restriction in the form of a valve that can bepartially opened to allow increased flow or partially closed to decreaseflow. The opening of the valve is controlled by a closed loop feedbackcircuit that minimizes the difference between an externally provided setpoint and the reading from an internal flow measuring device. The flowmeasuring device uses a thermal sensor with two resistance-thermometerelements wound around the outside of a tube through which the gas flows.The elements are heated by applying an electric current. As the gasflows through the tube, it picks up heat from the first element andtransfers it to the second element. The resulting temperaturedifferential between the two elements is a measure of the mass flow rateof the gas. In the newer, pressure insensitive MFCs, a pressuretransducer is included between the thermal sensor and the control valveto account for the effects of changing pressure on flow.

A consequence of the thermal sensor flow measurement used in the MFC isthat accurate flow control requires regular calibration of the device.Without regular calibration, the actual flow rate through the MFC candrift to unacceptable values due to errors in the flow measuring device.This calibration is often performed by flowing gas through the MFC intoor out of a known volume and measuring the pressure rise or drop in thevolume. The actual flow rate can be determined by calculating the rateof pressure rise or drop and using establishedpressure-temperature-volume gas relations. This type of measurement isknown as a rate-of-rise calibration.

The rate-of-rise flow calibration is based on primary flow measurements,and is therefore a primary calibration standard—that is, flow isdetermined only by measurements of mass, pressure, volume, and time.There are only three types of known primary flow measurements:gravimetric, measuring the change in mass over time; volumetric,measuring the change in volume at constant pressure over time; andrate-of-rise, measuring the change in pressure at constant volume overtime. All other types of flow measurement are secondary measurements andmust be calibrated to a primary measurement.

Another method of metering the flow rate of a gas is to vary thepressure of the gas upstream of a critical orifice. The volume-flow rateof a gas through a critical orifice at constant temperature isindependent of the upstream or downstream pressure, provided thatcertain pressure requirements are met, e.g., the upstream pressure istwice that of the downstream pressure. By controlling the density of theupstream gas, which is proportional to pressure, the mass-flow ratethrough the critical orifice can be controlled.

In this type of flow control, the pressure is controlled using a controlvalve in a closed loop control circuit with a pressure transducerpositioned between the control valve and the critical orifice. Thecontrol valve is opened or closed to maintain a specified pressureupstream of the critical orifice. Mass flow rate is determined from thepressure upstream of a critical orifice and the establishedcharacteristics of the critical orifice. Accurate flow metering,therefore, is dependent not only on the performance of the pressurecontrolling system, but also on the mechanical integrity and stabilityof the dimensions of the orifice. Since the orifice is susceptible tobeing restricted with particulate contamination or eroded with reactionby the gases flowing through it, it is desirable to calibrate thepressure-flow relationship on a regular basis. This is performed usingthe same rate-of-rise measurement that is used for the MFC.

Both of the above mentioned methods control mass flow using a closedloop control scheme in which mass flow is ultimately the result of apressure gradient acting across a flow restriction. Viewed as a controlsystem, the output variable of these devices is mass flow, and the inputvariables are pressure and flow restriction characteristics.

In the case of the MFC, it controls the dimensions of the flowrestriction based on a second-order measurement of mass flow rate. Theactual dimensional characteristics of the flow restriction are unknown,but can be adjusted proportionally to increase or decrease flowrestriction as desired. In terms of the process variables, flowrestriction and pressure, only pressure is observable by the device (forthe pressure-insensitive MFCs), and only the flow restriction can becontrolled.

The critical orifice device controls flow by monitoring and controllingthe upstream pressure while maintaining presumably constant flowrestriction characteristics. The critical orifice device does notmonitor or control the characteristics of the flow restriction beyondassuming they are constant. In terms of process variables, pressure isboth observable and controllable by the device, while flow restrictionis not controllable or observable. It is true that without any externalinfluence, the characteristics of the flow restriction should not varywith time; however, in operation, the possibility exists for eitherchemical or mechanical perturbation of the flow restriction. This typeof perturbation cannot be measured by the system, and therefore, cannotbe corrected without the aid of an external calibration.

The shortcomings of both of these flow control schemes, especially theneed for external measurements for calibration and detection of faults,illustrate why an improved flow control scheme is desirable.

A key requirement of a flow control device that is able to detect faultsin its operation as well as to correct those faults throughself-calibration is that there be a sufficient number of processvariables that are observable and controllable. For both types of flowcontrol devices described above, which together comprise the vastmajority of flow control devices used in the semiconductor industry,there are not sufficient process variables to accomplish these tasks.

In the present invention, these process variables are added byimplementing a control valve that is designed to provide a highlyaccurate and repeatable mapping between its position and its flowrestriction characteristics, and is able to achieve a very accuratemeasurement and control of its position.

If the flow restriction can be controlled and also measured, the onlyadditional input necessary to control the flow rate is knowledge of thepressure gradient acting on the flow restriction, because flowconductance is a knowable, repeatable function of the flow restrictiondimensions. This control scheme is similar to that of the criticalorifice device, except the static flow restriction is replaced with onethat is variable and measureable.

The benefits obtained through use of this controllable valve will dependon the type of flow control device in which it is implemented. For thecritical orifice flow control device, substitution of the controllablevalve for the critical orifice will remove the uncertainty of any changeto the dimensions of the critical orifice. For the thermal sensor MFC,since the combination of the pressure transducer and controllable valveprovides a known flow rate, this flow rate can be checked against theflow rate measured by the thermal sensor, where any discrepancy is notedas a fault.

Neither of these flow control device types, however, allowsself-calibration during operation. For that capability, the flow controldevice must incorporate a primary flow measurement as an integral partof its operation. Incorporating this type of control valve into the flowmonitoring system shown in FIG. 1 yields a flow control device that ishighly accurate and self-calibrating. Initially, the position of thecontrol valve 108, which determines the flow restriction, would becontrolled based on a rate-of-drop flow measurement carried out withvalve 106 fully closed. Following the rate-of-drop measurement, valve106 would be opened and flow would be controlled by adjusting theposition of the control valve based on the pressure in the system asmeasured with pressure transducer 112.

A flow restriction with measurable, controllable dimensions is a keypiece to a greatly improved flow control scheme, which can ultimatelylead to a flow control device that is self calibrating and does not relyon secondary flow measurements. The technological challenges in makingthis type of control valve that is accurate enough for the semiconductorindustry become apparent from an order-of-magnitude estimate of theprecision required. The mass flow accuracy currently required forsemiconductor processing equipment is +/−1%. In general, flow must becontrolled between 1 and 10,000 sccm (standard cubic centimeters persecond), and the pressure difference between the flow restriction inletand outlet is typically between 20 and 150 psi (pounds per square inch).If we imagine an illustrative flow restriction in the form of arectangle with a static width of 10 mm and a static length of 1 mm, witha 60 psi pressure drop across the restriction, the height must beadjusted to 0.8 um to allow flow of 1 sccm. Performing error propagationwith the specification of +/−1%, the height must be controlled to within+/−1.1 nm.

In fact, except for high precision metering valves, which are not wellsuited for the cost, space, cleanliness, and reliability requirements ofsemiconductor manufacturing, very little work has been done to implementcontrol valves with measureable and controllable restrictions. U.S. Pat.No. 6,761,063-B2, entitled “True Position Sensor for Diaphragm Valves,”uses “a thin conductive member disposed between the diaphragm membraneand the actuator” to measure the capacitance between this conductivemember and the valve body. The capacitance value provides an indicationof the distance between the conductive member and the valve body, whichgives an indication of the distance between the diaphragm and the valvebody, which then gives an indication of the amount of valve opening.This type of assembly, with all of these separate parts, will at bestprovide control to a precision of approximately 0.001 inch, which isapproximately 25,000 nm. In addition, the inventors note that thecapacitance will change when fluid flows through the valve, thus makingthis approach even less appropriate for measuring the characteristics ofthe flow restriction to the level of accuracy needed in the presentinvention.

SUMMARY

The following summary of the invention is included in order to provide abasic understanding of some aspects and features of the invention. Thissummary is not an extensive overview of the invention and as such it isnot intended to particularly identify key or critical elements of theinvention or to delineate the scope of the invention. Its sole purposeis to present some concepts of the invention in a simplified form as aprelude to the more detailed description that is presented below.

Embodiments of the present invention provide for a controllable flowrestriction in which the dimensions of the flow restriction aremeasurable and controllable to a very high degree of precision. Themeasurement and control of the dimensions are precise enough that theycan be used to accomplish the self-calibrating gas-flow-control schemeshown in FIG. 1 with the flow accuracy required by the semiconductorindustry.

Embodiment of the invention provide an apparatus for controlling theflow of fluid, comprising: a first block having a flow restrictionsurface; a second block having a complementary flow restriction surface,wherein the flow restriction surface and the complementary flowrestriction surface cooperate to form a flow restriction valve; a fluidinlet hole formed in one of the first block or second block andproviding fluid passage to the flow restriction valve; a fluid outlethole formed in one of the first block or second block and providingfluid passage to the flow restriction valve; a seal provided about theflow restriction valve; and, wherein a change in the amount of the flowrestriction valve opening is effected by elastic flexure of at least oneof the first block or second block.

Embodiments of the invention also provide for a system for precision gasdelivery, comprising: a flow control valve; a pressure transducermeasuring gas pressure upstream of the flow control valve; a temperaturesensor; a flow regulator positioned upstream of the flow control valve;a conduit coupling the flow regulator to the flow control valve; and, acontroller receiving signals from the pressure transducer andtemperature sensor and controlling the operation of the flow controlvalve according to flow calculation; wherein the flow control valvecomprises an actuator varying the amount of gas flow by elasticallyflexing a body part of the flow control valve.

Embodiments of the invention also provide for a method for controllingflow rate through gas delivery system having a flow control valve, aflow regulator, and a known gas confinement volume coupled between theflow regulator and the flow control valve; comprising: actuating theflow control valve to deliver a desired flow rate; temporarilyinterrupting gas flow through the flow regulator; measuring temperatureof the gas; measuring pressure drop of the gas within the known volume;using the measured temperature, pressure drop and known volume tocalculate flow rate through the flow control valve; resuming flowthrough the flow regulator; and, using calculated flow rate to adjustthe flow through the flow control valve by activating an actuator tocause elastic flexure of a body part of the flow control valve.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, exemplify the embodiments of the presentinvention and, together with the description, serve to explain andillustrate principles of the invention. The drawings are intended toillustrate major features of the exemplary embodiments in a diagrammaticmanner. The drawings are not intended to depict every feature of actualembodiments nor relative dimensions of the depicted elements, and arenot drawn to scale.

FIG. 1 is a simplified schematic diagram of an embodiment of anapparatus in accordance with the present invention for self-calibratinggas flow control.

FIG. 2 is a simplified schematic diagram of an embodiment of anapparatus in accordance with the present invention for a high precisioncontrollable flow restriction, while FIG. 2A illustrates an alternativeembodiment.

FIGS. 2B-2D illustrate examples for modifications of the embodiment ofFIG. 2, while FIG. 2E illustrates an enlarged view of details of FIG.2D.

FIGS. 2F and 2G illustrate another embodiment of the invention.

FIG. 3 is a simplified schematic diagram showing the details of the flowrestriction.

FIG. 4 is a simplified schematic diagram showing the embodiment of FIG.2 with the flow restriction opened, where the amount of opening isdesignated by “h”.

FIG. 5 is a graph showing the relationship between flow and the amountof flow restriction opening, h, for the specified inlet pressure andflow restriction radial dimensions.

FIG. 6 is a simplified schematic diagram of an embodiment of anapparatus in accordance with the present invention for a high precisioncontrollable flow restriction, including a processor and computerreadable storage medium to control automatically the amount of flowrestriction opening.

DETAILED DESCRIPTION

Embodiments of the present invention provide for a controllable flowrestriction in which the dimensions of the flow restriction aremeasurable and controllable to a very high degree of precision. Themeasurement and control of the dimensions are precise enough that theycan be used to accomplish the self-calibrating gas-flow-control schemeshown in FIG. 1 with the flow accuracy required by the semiconductorindustry.

In various embodiments of the present invention, this level of precisionis obtained by incorporating the following characteristics:

-   -   1. Uniaxial motion of the two opposing faces of the flow        restriction, where transverse and/or rotational motion in the        other two axes is limited to less than approximately 1 nm;    -   2. Measurement of motion in the uniaxial dimension to a        precision of approximately 1 nm;    -   3. Actuation of motion with resolution of approximately 0.1 nm.

An illustrative embodiment of the invention, shown in FIG. 2, consistsof two adjacent bodies 201 and 202 with a planar contacting area thatforms the flow restriction valve 211. That is, the first block 201 has aflow restriction surface 213 and the second block 202 has acomplementary flow restriction surface 212. The flow restriction surface213 of the first block 201 cooperates with the complementary flowrestriction surface 212 of the second block 202 to thereby form flowrestriction valve 211. In the embodiment of FIG. 2, the flow restrictionvalve 211 is formed by an annular extension 215 formed on the flowrestriction surface 213 of block 201, thereby defining hole 216 (see,FIG. 3). Conversely, the complementary flow restriction surface 212 ismachined to be flat so as to form a perfect seal when urged against theannular extension 215.

The first body or block 201 is static in space, and the second body orblock 202 is coupled to the first with a cantilever 203. The cantileveris positioned so that the motion of the second body with respect to thefirst at the planar contacting area is essentially uniaxial and verypredictable and reproducible. The planar faces of the bodies arepatterned to form two separate cavities 204 and 205 that are isolatedfrom each other when the two bodies are contacting, but are coupled by aflow restriction valve 211 when the bodies are displaced from eachother.

In FIG. 2, bodies 201 and 202 are machined from a single piece ofmaterial, such as, e.g., stainless steel. Of course, any material thatis compatible with the gas being used and allows reliable and repeatableflexure at the cantilever could be used. For example, alternativematerials include other types of steel, Inconel, Hastelloy, etc. It isnoted, however, that when made from one solid piece of material, itwould be difficult to machine the valve surfaces. Therefore, FIG. 2Aillustrates an alternative embodiment, wherein bodies 201 and 202 arefabricated from two pieces, so that the machining becomes verystraightforward. Of course, the one or both bodies 201 and 202 may bemade from more than one single piece. The most typical way to fasten thepieces together is with fastener 220, such as, e.g., bolts, but theycould also be glued or welded together. The main requirement for thefastening is that there should be no movement at the location of thefastening, but it should allow for uniaxial movement at the flowrestriction area. As both embodiments of FIGS. 2 and 2A would operatethe same, the following description proceeds as applicable to eitherembodiment.

An actuator 206 is installed in the first body 201 which acts on thesecond body 202 to induce displacement of the second body, and thereforechange the flow restriction dimension. That is, as the actuator expandsor contracts, it causes an elastic flexure in body 202 about thecantilever 203. This is similar to what is sometimes referred to asflexure bearing, wherein the motion is caused by elastic flexure ordeformation of the material forming the flexure bearing. Since themotion is elastic deformation, it is very precise and controllable.Also, when relaxed, the apparatus inherently assumes its naturalposition due to the elastic nature of the deformation. The displacementsensor 207 is installed in the first body to measure this displacement.In one embodiment, this is accomplished using a capacitive measuringdevice, or displacement sensor, which can measure linear displacementson the order of one nanometer.

A closed loop control circuit is formed with the output of the sensor207 and the action of the actuator 206 to accomplish control of the flowrestriction 211 dimensions, and consequently, the flow conductancecoupling the two cavities. Piping 208 and 209 is incorporated into thesystem such that gas flow is directed through hole 218 into one cavityand out of the other cavity 205 through hole 219 to pipe 209, such thatall flow must pass through the flow restriction valve 211 defined by thetwo bodies.

By coupling the two bodies that form the flow restriction with acantilever, as opposed to mechanical hinges or sliding assemblies,mechanical play and hysteresis are eliminated because friction sourcesare eliminated. Also, during actuation, there is negligible elasticdeformation within the two bodies; elastic deformation is isolated tothe cantilever coupling the two bodies. Both planes which define theflow restriction, therefore, are rigid.

As depicted in FIGS. 2 and 2A, body 202 is positioned as close aspossible to body 201, thus closing the flow restriction 211. Accordingto one embodiment, the two bodies 201 and 202 are constructed such thatwhen they are coupled together via the fastener, the two bodies areurged against each other so as to close the flow restriction 211. Aflexible seal 210 is provided about the flow restriction 211, so as toprevent gas flow to the atmosphere. Also, according to one embodiment,the seal 210 is constructed such that it is in tension and serves topull the two bodies 201 and 202 together to cause the flow restriction211 to be closed. In this particular figure, the linear actuator 206,which is secured in body 201 and pushes against body 202, is in itsrelaxed state. When the linear actuator 206 is activated, it pushesagainst the tension of the flexible seal 210, moving body 202 away frombody 201, forcing body 202 to pivot on the cantilever 203 andconsequently allowing the flow restriction 211 to open up. It should beappreciated that the actuator 206 may be attached to body 202 and pressagainst body 201.

This flow restriction 211, as viewed from the top, forms a circle asshown in FIG. 3, where the inside dimension of the flow restriction 211is r₁ and the outside dimension is r₂. Gas flow is from the inside ofthe flow restriction 211, across the flow restriction 211, to theoutside of the flow restriction 211. That is, with reference to FIG. 4,gas flows from inlet piping 208, to cavity 204, to cavity 205 (when theflow restriction 211 is open) and, since it's blocked by seal 210,proceeds to outlet piping 209. Consequently, when the two bodies 201 and202 are closed against each other as shown in FIG. 2, the flowrestriction 211 is closed, and no gas can flow. When the linear actuator206 is activated, the flow restriction 211 opens up, and gas can flowfrom the inlet 208 to the outlet 209. In general, the flow of gas willincrease as the flow restriction opens.

Since both body 201 and body 202 are rigid, and the only motion that canoccur in the apparatus is flexure of the body at the cantilever 203, themovement of body 202 with respect to body 201 is very well defined. Forsmall movements, where the opening of the flow restriction 211 is on theorder of micrometers, which is much smaller than the distance betweenthe flow restriction and the flexure, the movement of body 202 withrespect to body 201 at the flow restriction will be essentially uniaxialin a direction perpendicular to the plane of the flow restriction 211.This well defined movement is critical for reproducible gas flowcharacteristics of the apparatus.

As can be appreciated, the embodiment of FIG. 2 is provided only as oneillustration and it may be varied without detracting from itseffectiveness. For example, FIG. 2B illustrates an embodiment whereinone body, here body 201, includes the hole for the gas inlet, while theother body, here body 202, includes the hole for the outlet. Of course,the reverse can also be done with the same result. FIG. 2C illustratesan embodiment wherein the two bodies are not connected to each other.Rather, body 201 is anchored and does not move, while body 202 isanchored independently via a cantilever arrangement, such that it can beelastically flexured to control the opening of the flow restriction.

Additionally, FIGS. 2D and 2E illustrate how the seal can be implementedsuch that it is also controlling the amount of flow through the valve.As shown in FIG. 2D and in the detail view of FIG. 2E, seal 210 isprovided about cavities 204 and 205. Its periphery is fixedly attachedto stationary body 201, while its central region is fixedly attached toflexure body 202. In this manner, when actuator 206 is actuated, itpulls on body 202, which in turn pulls on seal 210. Consequently, seal210 elastically deforms such that it creates an opening of height “h” toenable gas to flow from cavity 204 to cavity 205.

FIGS. 2F and 2G illustrate yet another embodiment of the invention,wherein FIG. 2F illustrates the closed, i.e., no flow condition, andFIG. 2G illustrates the open position. As shown, body 202 is joined tobody 201 via flexures 221. In one embodiment, body 201 and body 202 arecylindrical and the flexure parts 221 are round disks extending frombody 202 and may be machined from the same block as body 202 or maybesimply attached to body 202 by, e.g., welding. While other shapes arepossible, circular shapes would provide uniform and balanced movement.In this embodiment, the lower flexure part 221 also functions as theseal 210, although it is clearly possible to provide a separate seal,such as with the other embodiments. Linear actuator 206 is providedbetween lever 240 and the top portion of body 201, such that when theactuator 206 expands, it raises the lever so as to raise body 202 andelastically flex the flexure parts of body 202, as illustrated in FIG.2G. In the elevated position, the bottom surface of body 202, whichforms the flow restriction surface, is raised a distance “h” from thecomplementary flow restriction surface of body 201, to thereby allowcontrolled fluid flow through the flow restriction valve 211. In thisembodiment the two cylindrical flexures would limit relative motionbetween the bodies 201 and 202 to one degree of freedom (vertical), andwould restrict rotation of the bodies with respect to each other in theplane of the page. This enables high accurate control of the fluid flowthrough the flow restriction 211.

If we quantify the amount of flow restriction opening as “h”, as shownin FIG. 4, we can write the following equation for the flow of gas as afunction of the opening, h:

Flow=2πP _(in) ² h ³/3RTμ ln(r ₁ /r ₂)  Equation (1)

whereP_(in) is the pressure of the gas at the inlet 208R is the universal gas constant=1.986 calories per mol per KT is the absolute temperature in Kμ is the viscosity of the gasand h, r₁, and r₂ are the dimensions shown in FIGS. 3 and 4.

For most gas flow applications, Equation (1), which describes laminarflow through the flow restriction, will provide a sufficiently accurateanswer; however, for those cases where the downstream pressure, i.e.,the pressure of the gas at the outlet 209, P_(out), is sufficiently highcompared to the pressure, P_(in), at the inlet 208, the flow determinedin Equation (1) must be multiplied by cos(arcsin(P_(out)/P_(in)).

FIG. 5 shows the gas flow for an inlet pressure of 0.2 MPa(approximately 30 psi), absolute. One of the advantages of theconfiguration of the restriction is that the flow is a function of thecube of the restriction opening, h. This means that one order ofmagnitude change in the amount of flow restriction opening can controlthree orders of magnitude of flow, giving the apparatus a very largerange of flow rate control.

The linear actuator 206 can be of various types, such as a solenoid orpiezoelectric actuator. A typical example is a piezoelectric actuator,part number P830.30, from Physik Instrumente, GmbH ofKarlsruhe/Palmbach, Germany. The displacement sensor can also be ofvarious types, such as a strain gauge or capacitance position sensor. Atypical example is a capacitance position sensor, part number D510.050,also from Physik Instrumente.

To be useful as a gas flow controller, the apparatus of FIG. 2 must havesome means to control the amount of the flow restriction opening, h.FIG. 6 shows such an embodiment, with a controller 601 that measures theoutput of the displacement sensor, and using values stored in thecomputer readable storage medium, determines the amount of flowrestriction opening, h. The controller then controls the linear actuatorto move body 202 until the value indicated by the displacement sensor isconsistent with the desired opening, i.e., the position set point. Thiscontrol can be carried out with a standard control loop, such as a PID(proportional-integral-derivative) controller.

As indicated by Equation (1), in addition to the known values of h, r₁,and r₂, effective control of the gas flow rate also requires that P_(in)and T be known. The determination of these parameters can be carried outwith the apparatus shown in FIG. 1. In this embodiment, the apparatus600 of FIG. 6 is represented by the control valve 108 of FIG. 1. Thecontroller 601 of FIG. 6 is part of the control valve 108 of FIG. 1 andrepresents a control loop that is nested within the control loop ofcontroller 120 of FIG. 1.

The controller 120 of FIG. 1 has stored within its computer readablestorage medium the values that allow it to determine the required amountof flow restriction opening, h, that is necessary to obtain the desiredflow rate for a given gas pressure and temperature. The determination ofthe required opening can be carried out using an equation such asEquation (1) or alternatively, using a lookup table that is determinedahead of time by measuring the gas flow rate for a wide range of valuesof P_(in), T, and h.

The gas flow controller of FIG. 1 has a sufficient number of observableand controllable parameters to be able to perform self-diagnostics andself-calibration. Furthermore, these self-diagnostics andself-calibration can take place while the gas flow controller isdelivering gas at a desired flow rate to a process chamber.

As shown in FIG. 1, the apparatus comprises a gas line 101 having aninlet 103 in fluid communication with a gas source 104, and an outlet105 in fluid communication with a process chamber (not shown). Understandard process conditions, the valve 106 would be open and gas wouldbe flowing through the volume 110, through the control valve 108, andthen ultimately into the process chamber.

The volume 110 represents the total fixed volume between the valve 106and the control valve 108. A pressure transducer 112 is configured tomeasure the pressure in this volume V 110. A temperature sensor 114 ispositioned to measure the temperature of the components. In certainembodiments, the sensor 114 may be a specialized sensor in directthermal communication with one or more components. In other embodiments,where the environment is temperature-controlled and it is not expectedthat the temperature will vary greatly from place to place or time totime, a thermometer positioned near the gas delivery system will providesufficient information regarding the temperature of interest.

The procedure for testing the flow of gas through the control valve 108may be summarized as follows:

1. The control valve 108 is set to a desired flow rate, and a flow ofgas is established.2. The valve 106 is closed.3. While the valve 106 is closed, the pressure is measured at regularperiods, typically ranging from 1 to 100 milliseconds, by the pressuretransducer.4. After the pressure has dropped by some amount (typically 1-10% of thestarting value), the valve 106 is opened, and the testing procedureconcluded.5. At some point during this measurement, the reading of the temperaturesensor 114 is noted.There is some amount of flexibility in the ordering of these steps; forexample, steps 1 and 2 can be interchanged. Step 5 can be done at anytime during the testing procedure.

Some elaboration on the valve 106 is warranted. In its simplest form,valve 106 would be an on/off shutoff valve. A potential disadvantage ofthis type of valve is that in step 4, when the valve is opened, therewill be a rapid rise in pressure inside the volume V 110. This rapidrise in pressure might make it difficult for the control valve 108 tochange the amount of flow restriction opening sufficiently fast to keepa constant flow of gas flowing to the process chamber. A goodalternative to the shutoff valve is a metering valve (as indicated inFIG. 1), which is a valve designed to provide varying gas flow ratesover a range of settings. When metering valve 106 is opened at the endof the measurement period, the controller controls the amount of valveopening such that the rise in pressure, as determined with pressuretransducer 112, is maintained at a certain rate that is sufficiently lowso that the flow through the control valve 108 is not perturbed. Inother words, the opening of metering valve 106 is performed graduallyrather than abruptly, so that the gas flow is not perturbed.Alternatively, rather than raising the pressure at all during theprocess step, the pressure could be held constant at the end of themeasurement period and then raised once the process step was terminated.This approach would have the least effect on any perturbation of theflow rate through the control valve 108.

According to the ideal gas equation, the amount of gas in the volume V110, is given by:

n=PV/RT,  Equation (2)

wheren=amount of gas (measured in moles)P=pressure measured by the pressure transducerV=volume of gasR=ideal gas constant=1.987 calories per mol per KT=absolute temperature in K.

To some extent, all real gases are non-ideal. For these non-ideal gases,Equation (2) can be rewritten as:

n=PV/ZRT, where  Equation (3)

Z=compressibility factor.

The compressibility factor can be found in various handbooks or it canbe determined from experimental measurements for any particular gas, andis a function of temperature and pressure.

The flow rate of a gas can be written as the change in the amount of gasper unit time; i.e.:

flow rate=Δn/Δt,  Equation (4)

where t=time.

Substituting into Equation (4) from Equation (3), yields:

flow rate=(ΔP/Δt)V/ZRT.  Equation (5)

The first factor (ΔP/Δt) is merely the slope of the pressuremeasurements as a function of time taken in step 3 of the procedureabove. Thus, taking these pressure measurements in conjunction with thevolume, temperature, and the compressibility factor, the actual rate offlow of the gas through the control valve 108 can be determinedaccording to embodiments of the present invention, thus providing twoindependent measurements of the gas flow rate into the process chamber.

One or more steps of the various embodiments of the present inventioncould be performed with manual or automatic operation. For example, thesteps of opening/closing valves and taking pressure readings could beconducted automatically according to computer control. Alternatively,one or more of the various valves could be actuated manually, with theresulting flow rate calculated automatically from the detected pressuredrop. Automatic operation of one or more steps could be accomplishedbased upon instructions stored in a computer readable storage medium,utilizing communication through control lines as indicated in FIG. 1.

Another benefit of this measurement system is that if a discrepancy isfound between the desired flow rate and the measured flow rate, thesetting of the control valve 108 can be changed to correct for thediscrepancy and provide the desired flow rate. This type of correctionis particularly appropriate considering that the pressure rate-of-dropmeasurement provides a primary calibration standard. This correction canbe done in the same process step or in a subsequent process step. Thistype of correction is greatly simplified if the system is under computercontrol.

It should be understood that processes and techniques described hereinare not inherently related to any particular apparatus and may beimplemented by any suitable combination of components. Further, varioustypes of general purpose devices may be used in accordance with theteachings described herein. It may also prove advantageous to constructspecialized apparatus to perform the method steps described herein.

The present invention has been described in relation to particularexamples, which are intended in all respects to be illustrative ratherthan restrictive. Those skilled in the art will appreciate that manydifferent combinations of hardware, software, and firmware will besuitable for practicing the present invention. Moreover, otherimplementations of the invention will be apparent to those skilled inthe art from consideration of the specification and practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with a true scope and spiritof the invention being indicated by the following claims.

What is claimed is:
 1. A gas flow control valve, comprising: a firstbody maintained static in space; a second body movably situated insidethe first body; a flow restriction valve formed by a flow restrictionsurface provided on the first body and a complementary flow restrictionsurface formed on the second body; a lower flexure part extending fromthe second body and coupling the first body and the second body and asecond flexure part positioned above the lower flexure part and couplingthe first body and the second body, wherein the lower flexure part andthe second flexure part limit relative motion between the first body andthe second body, thereby permitting only uniaxial motion between thefirst body and the second body; a lever connected to the second body; anactuator provided between the lever and the first body, such that whenthe actuator expands, it raises the lever so as to raise the second bodyand elastically flex the lower and second flexure parts thereby inducingdisplacement between the first body and the second body; and, adisplacement sensor installed to measure the displacement between thefirst body and the second body.
 2. The gas flow control valve of claim1, further comprising an annular extension machined to form a perfectseal between the flow restriction surface and the complementary flowrestriction surface.
 3. The gas flow control valve of claim 1, furthercomprising first cavity and a second cavity formed between the flowrestriction surface and the complementary flat flow restriction surface,and wherein when the actuator is not energized, the flow restrictionsurface and the complementary flat flow restriction surface preventfluid flow between the first cavity and second cavity.
 4. The gas flowcontrol valve of claim 1, further comprising inlet piping and outletpiping, and wherein the first cavity is coupled to the inlet piping andthe second cavity is coupled to the outlet piping.
 5. The gas flowcontrol valve of claim 1, wherein the actuator is configured to inducedisplacement between the first body and the second body in a directionseparating the flow restriction surface and the complementary flowrestriction surface.
 6. The gas flow control valve of claim 1, whereinthe displacement sensor can measure linear displacements on the order ofone nanometer.
 7. The gas flow control valve of claim 1, wherein thedisplacement sensor is a capacitance position sensor.
 8. The gas flowcontrol valve of claim 1, further comprising a lookup table correlatingrequired displacement and fluid flow through the flow control valve. 9.The gas flow control valve of claim 1, further comprising a closed loopcontrol circuit formed with an output of the displacement sensor andaction of the actuator.
 10. The gas flow control valve of claim 1,further comprising a controller that measures output of the displacementsensor, and using values stored in a computer readable storage medium,determines an amount of flow restriction opening, and controls thelinear actuator to move the second body until a value indicated by thedisplacement sensor is consistent with a desired opening.
 11. The gasflow control valve of claim 10, wherein the controller executesproportional-integral-derivative control.
 12. The gas flow control valveof claim 1, wherein when the actuator expands it causes deformation ofthe lower flexure part and the second flexure part, thereby causinguniaxial motion of the second body in a direction perpendicular to planeof the complimentary flow restriction surface.
 13. The gas flow controlvalve of claim 1, wherein the actuator comprises a piezoelectricactuator.
 14. The gas flow control valve of claim 1, further comprisinga controller determining a required displacement using a lookup tablethat is predetermined by measuring gas flow rates for a wide range ofvalues of input gas pressure, gas temperature, and displacement signalfrom the displacement sensor.
 15. The gas flow control valve of claim 1,wherein the lower flexure part and second flexure part are configured toelastically deform when the actuator induces displacement between thefirst body and the second body.
 16. The gas flow control valve of claim1, wherein the displacement sensor measures the uniaxial motion withresolution of at least 1 nanometer.
 17. The gas flow control valve ofclaim 1, wherein the actuator is configured to induce displacementbetween the first body and the second body with a resolution of at least0.1 nanometer.
 18. The gas flow control valve of claim 1, wherein thelower flexure part forms a seal shaped as a round disk.
 19. The gas flowcontrol valve of claim 1, wherein either the first or the second flexurepart forms a seal.
 20. The gas flow control valve of claim 1, whereinthe actuator is provided between the lever and a top portion of thefirst body and the displacement sensor is installed on the first body.